1. Field of the Invention
The present invention relates to a liquid chromatograph and a sample introducing apparatus therefore.
2. Description of the Related Art
A liquid chromatograph, which is shown in FIG. 1, is equipped with an analytical flow path consisting of an eluent holder 1, an eluent feeder 2, a separator 4, and a detector 5. The eluent feeder 2 feeds an eluent from the eluent feeder 2 to the separator 4.
The analytical flow path has a sample injector 3 placed between the eluent feeder 2 and the separator 4 (including a separation column). The sample injector 3 injects a sample into the flowing eluent, so that the sample is separated into components in the separator 4 and the separated components are detected by the detector 5.
The liquid chromatograph is usually constructed of several units, such as a pump (as the eluent feeder 2), an autosampler (as the sample injector 3), a separation column (as the separator 4), and a detector (as the detector 5).
The autosampler as the sample injector 3 is recently dominated by that of direct injection type. The autosampler of direct injection type causes the inside of the needle to constitute a part of the analytical flow path while the eluent is being fed under a high pressure for analysis.
The autosampler of direct injection type injects a sample in the following manner. First, the needle is detached from the analytical flow path under high pressure, and the detached needle functions as a part of the sample introduction flow path in the autosampler. The sample introduction flow path, which is connected to a syringe to suck up and discharge a sample in the autosampler, sucks up a sample through the needle as the plunger of the syringe is pulled out. After sample sucking, the sample introduction flow path, which holds the sample sucked up through the needle, is switched such that the analytical flow path for the eluent to flow from the pump communicates with the analytical flow path connected to the separation column and the sample held in the sample introduction flow path is injected into the analytical flow path. The autosampler of direct injection type employs a valve to switch the analytical flow path and the sample introduction flow path. It is common practice to place one valve each in the flow paths upstream and downstream the needle.
An ordinary autosampler of direct injection type performs sample injection in a manner which is explained below with reference to FIGS. 2 and 3.
In an ordinary autosampler of direct injection type, the flow path takes the route shown in FIG. 2 when a sample is discharged or sampling is not performed. The eluent delivered from the pump passes through the valve A 14 and then the needle 12. The eluent passes further through the injection port 13 and the valve A 14 again and flows toward the separation column.
Also, in an ordinary autosampler of direct injection type, the flow path takes the route shown in FIG. 3 when a sample is sucked up. At the time of sample suction, the valve A 14 acts to switch the flow path. The pump's action causes the eluent being delivered from the pump to flow directly to the separation column without passing through the needle 12 and the injection port 13. After separation from the flow path to the separation column, the needle 12 and the sample loop 22 are connected to the syringe 20 which moves the liquid in the autosampler. With the needle 12 inserted into the vial 23, the syringe 20 sucks up the sample so that the sample is held in the needle 12 and the sample loop 22.
After sample sucking, the needle 12 is connected to the injection port 13, and then the valve A 14 is switched again so that the flow path takes the previous route shown in FIG. 2. As the result, the eluent delivered from the pump enters the sample loop 22 and the needle 12, and the sample held in the needle 12 and the sample loop 22 is pushed into the injection port 13. Thus, the sample flows through the injection port 13 and the valve A 14 to reach the separation column.
As mentioned above, an autosampler of direct injection type is constructed such that both the upstream side and the downstream side of the needle are connected to the valve. This construction is disclosed in Patent Documents 1 and 2 below.    Patent Document 1:            Japanese Patent Laid-open No. 2007-121192            Patent Document 2:            Japanese Patent Laid-open No. 2005-265805        
The autosampler of direct injection type, which is used for the sample injector 3, has a valve to switch the analytical flow path and the sample introduction flow path, the valve being placed at the upstream and downstream sides of the needle.
The disadvantage of the autosampler of direct injection type mentioned above is the necessity of more than one valve. Valves increase the dead volume which deteriorates the accuracy of analysis.
“Dead volume” means any space which exists in the flow from sample introduction to detection in the liquid chromatograph and which is useless, or rather harmful, to sample separation and hence is undesirable.
The dead volume has the following two adverse effects.
First, the dead volume diffuses the sample in the eluent, which results in the broadening of the detected peak.
The diffusion of the sample also reduces the theoretical plate number N (defined below) which is an index to represent the separation characteristics of the column.N=5.54×tR2/W0.5h2 where, tR′: retention time
W0.5h2: peak width at the middle of peak height
Moreover, the diffusion of the sample might mix again the zones of compounds purposely separated by the column.
Second, the dead volume increases the amount of carry-over or tends to cause carry-over.
“Carry-over” means a previously analyzed sample that remains in the liquid chromatograph to be detected in the subsequent analysis.
This is attributable to the dead space that exists at the joint between the pipe and the part or the valve. (There is no complete adhesion between the surface of the inserted pipe and the surface of the pipe receiver, and the gap between the surfaces forms the dead space.) The injected sample remains in the dead space and the remaining sample leaks out during subsequent analyses.
For these reasons, one of the important factors to improve the accuracy of analysis is how to reduce dead space in the analytical flow path from the autosampler to the separation column and from the separation column to the detector.
The conventional method of achieving the foregoing object is by using as thin a pipe as permissible (from the standpoint of flow rate and pressure) for the analytical flow path and also by using parts with a minimum area in contact with the liquid (thereby eliminating unnecessary spaces).
However, there is a limit to reducing the inside diameter and length of the pipe and changing the shape of parts to reduce dead volume.